The invention relates to a glass funnel and a glass bulb for a cathode-ray tube for use in television reception or the like.
As shown in FIG. 13, for example, a glass bulb 11 for constituting a cathode-ray tube for use in television reception or the like comprises a glass panel (hereinafter, referred to as “panel”) 12 on which images are displayed, a glass funnel (hereinafter, referred to as “funnel”) 13 having the shape of a funnel which forms the back thereof, and a neck portion 14 in which, an electron gun is installed. The neck portion 14 is fusion bonded to a smaller opening portion of the funnel 13. The panel 12 has a face portion 12a which makes an image viewing area and a skirt portion 12b which extends generally perpendicularly from the periphery of the face portion 12a. As shown enlarged in FIG. 14, a seal edge surface 12b1 arranged on the end surface of the skirt portion 12b and a seal edge surface 13c1 arranged on a larger opening portion of the funnel 13 are joined to each other through a seal glass 15 for sealing.
The glass bulb 11 for a cathode-ray tube, formed as described above, is used as a vacuum vessel after installing an electron gun in the neck portion 14 and then evacuating inside thereof (the internal pressure after the evacuation is on the order of, e.g., 10−8 Torr). Consequently, the external surface of the glass bulb 11 undergoes a stress caused by the load of the atmospheric pressure (hereinafter, this stress will be referred to as “vacuum stress”). It is required that the glass bulb 11 has mechanical and structural strengths sufficient to resist a fracture resulting from this vacuum stress (vacuum fracture). That is, if these strengths are insufficient, the glass bulb 11 may cause fatigue fracture since it cannot endure the vacuum stress. In addition, if accompanied with such foreign factors as minute flaws on the external surface or the application of an impact load, the fatigue fracture is expected to proceed faster. Besides, in the step of fabricating the cathode-ray tube, the glass bulb 11 is raised to around 400° C. in temperature. The thermal stress resulting from the temperature rise and the vacuum stress may produce a synergistic effect toward fracture.
Since the glass bulb 11 is aspheric, the vacuum stress acts on the glass bulb 11 as compressive stress and tensile stress. These stresses have general distributions as shown in FIG. 15. Here, FIGS. 15(a), (b), and (c) show stress distributions in a minor-axis section, a major-axis section, and a diagonal-axis section, respectively. In these stress distribution diagrams, the regions indicated with inward arrows represent regions undergoing compressive stress, and the regions indicated with outward arrows regions undergoing tensile stress.
Glass structures are generally weaker to tensile stress than to compressive stress in fracture strength. In the glass bulb 11 for a cathode-ray tube, as a vacuum vessel, a fracture is easy to progress originating with the regions undergoing tensile stress that results from the vacuum stress (hereinafter, this stress will be referred to as “tensile vacuum stress”), namely, the regions extending from the periphery of the face portion 12a to the skirt portion 12b of the panel 12 and the regions around the seal edge surface 13c1 of the funnel 13. In particular, the seal edge surface 12b1 of the panel 12 and the seal edge surface 13c1 of the funnel 13 are joined through the seal glass 15 for sealing. Since this joint portion is a weak point in strength while the tensile vacuum stress peaks in the vicinity of the joint portion {FIGS. 15(a) and (b)}, preventive measures against the fracture originating with the joint portion are of importance. For such reasons, the conventional glass bulb 11 for a cathode-ray tube has been increased in thickness to secure necessary fracture strength.
Recently, flatter or larger screens are required to displays for television reception and the like. Based on this, cathode-ray tubes are also on the way to flattening or planarization. Accordingly, glass bulbs for a cathode-ray tube are getting farther from being spherical in shape than ever before, and the vacuum stress distribution is increasing in the degree of unevenness. Thus, the strength level required to the glass bulbs for a cathode-ray tube grows in severity. This results in a further increase in the thickness of the glass bulbs for a cathode-ray tube, accompanied with an increase in weight. The increase in the weight of the glass bulbs for a cathode-ray tube not only imposes an inconvenience on transportation, handling, and the like, but also causes an increase in the weight of the final products incorporating the cathode-ray tubes, thereby causing lower commercial values. In particular, large-sized glass bulbs for a cathode-ray tube are more prone to that tendency.
Under the foregoing circumstances, a weight reduction is desired of glass bulbs for a cathode-ray tube. Meanwhile, it is also important to secure strength sufficient to resist vacuum fracture since the flattening or planarization of the cathode-ray tubes has increased the degree of unevenness of the vacuum stress acting on the glass bulbs for a cathode-ray tube.